Method for recovering viscous petroleum

ABSTRACT

When supplemental oil recovery methods such as steam flooding are applied to viscous oil containing formations such as tar sand deposits, poor vertical conformants frequently results, particularly when the oil formation contains one or more zones having a permeability substantially less than other zones in the formation. Recovery efficiency is improved if the injection well is completed so as to establish separate communication means between the surface and each of the zones of different permeability. After injection of a thermal recovery fluid such as steam into the more permeable layer is initiated, injection of a hydrocarbon solvent, either gaseous or liquid, is initiated by the separate communication means into the less permeable zone, and from about .05 to about .5 pore volumes of solvent material is introduced into the less permeable zone. This is displaced with gas such as natural gas or low molecular weight gaseous hydrocarbons, and then steam injection into the less permeable zone will be maintained at a value less than the injection pressure into the more permeable zone for a period until the desired receptivity in the lower permeability zone has been obtained, after which the injection pressure in the two zones is essentially equalized.

Primary Examiner-James A. Leppink Attorney, Agent, or Firm-T. H. Whaley', C. G. Ries; Jack H Park 0 United States Patent [1 1 3,903,967

Woodward 1 Sept. 9, 1975 METHOD FOR RECOVERING VISCOUS steam flooding are applied to viscous oil containing PETROLEUM formations such as tar sand deposits, poor vertical [75] inventor: Charles D. Woodward, Houston, c9nformaqts freque'ltly results pamcularly whep the Tex oil formation contains one or more zones having a permeability substantially less than other zones in the [73] Assignee: Texaco lnc., New York, NY. formation. Recovery efficiency is improved if the injection well is completed so as to establish separate [22] Flled' Sept 1974 communication means between the surface and each [21] Appl. No.: 508,029 of the zones of different permeability. After injection of a thermal recovery fluid such as steam into the 52 us. Cl. 166/269 more permeable l f hydm' 2 carbon solvent, either gaseous or liquld, IS Initiated by [51] Int. Cl. E21B 43/16 the Separate Communication means into the less [58] Field of Search l66/269, 305, 274 meable Zone and from about 05 to about 5 p VOL Re'erences cued umes of solvent material is introduced Into the less permeable zone. This is displaced with gas such as nat- UNITED STATES PATENTS ural gas or low molecular weight gaseous hydrocar- 3,22l,8l0 12/1965 Marx l66/269 bong, and then steam injection into the less permeable 3 7 12/1967 Parker 66/269 zone will be maintained at a value less than the injec- 3,42l,583 l/l969 Koons 166/269 tion pressum into the more permeable Zone for a pep iod until the desired receptivity in the lower permeability zone has been obtained, after which the injection pressure in the two zones is essentially equalized.

ll Claims, 1 Drawing Figure METHOD FOR RECOVERING VlSCOUS PETROLEUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention concerns a method for recovering viscous petroleum from a subterranean. viscous petroleum-containing formation such as a tar sand deposit, particularly from a formation having at least two zones differing substantially from one another in permeability.

2. Description of the Prior Art There are many subterranean, petroleum-containing formations in various parts of the United States and elsewhere in the world which contain petroleum whose viscosity is so great that it cannot be recovered from the formation by normal recovery techniques even if the formation exhibits satisfactory permeability and if a driving force is applied to the petroleum. The most extreme example of such viscous petroleum-containing formations are the so called tar sand deposits, which are known to exist in Utah and in other western states of the United States and Canada. The petroleum contained in these formations is so viscous that it is essentially immobile at reservoir temperature and pressure and so some supplemental treatment must be applied to the petroleum in order to increase the mobility of the petroleum, i.e. decrease its viscosity so that it will flow to a production well if a driving force is applied to the petroleum.

One of the more popular processes commonly used today for recovering viscous petroleum from subterranean formations is steam flooding, in which steam is injected into one or more injection wells to pass through the formation and heat the petroleum contained therein so as to decrease its viscosity. Even though the viscosity of the bituminous petroleum contained in tar sand deposits is known to be in the order to millions of centipoise at formation temperatures, the viscositytemperature relationship of petroleum is exceedingly shapr and is about 2 centipoise at ZOO-300F. Accordingly, in its simples embodiment, steam flooding is accomplished by injecting steam into one well, which steam passes through the formation to a remotely located production well and steam or steam condensate is recovered from the production well. The thermal energy contained in the steam raises the temperature of the viscous petroleum to a point that it will flow, and ultimately petroleum production is obtained from the production well as a result of the heating effect of steam injection.

A problem that is frequently encountered in subterranean viscous petroleum formations, and which reduces the effectiveness of thermal recovery methods such as steam flooding, is associated with the occurrence in the formation of a plurality of layers, usually horizontally oriented, which layers have significantly different permeabilities from one another. In a simple case, if a formation is comprised of two adjacent layers, one of which having a permeability substantially greater than the other, and steam is injected into the full thickness of the formation, steam flow will be confined to the more permeable layer and essentially no steam will pass into the less permeable layer. As a consequence of this channeling of the steam into the more permeable formation layer, the vertical conformance of the steam drive is quite low because only the petroleum contained in the more permeable layer is contacted by the steam. Consequently, once the petroleum contained in a permeable streak has been recovered, no additional oil is recovered from the formation even though steam is injected into the formation for long periods of time. The injected steam will channel through the depleted zone, leaving the originally less permeable zone relatively uneffected and undvpleted.

In actual formations, there may be more than two zones of varying permeability, and there are frequently layers interposed between the permeable streaks which may be shale or other relatively impermeable material. If the shale layers are continuous such that there is no vertical communication between the different zones, the zones can be isolated and treated separately with no resultant loss of efficiency as a result of the differences in permeabilities between the zones. Most formations do not contain such complete isolation of the various petroleum-containing intervals, however, and even though there are stratas of impermeable shale or other minerals between petroleum saturated intervals, these shale streaks are seldom sufficiently continuous as to preclude any vertical communication between the zones.

In view of the foregoing discussion, it can be appreciated that there is a substantial, unfulfilled need for a method of conducting a supplemental oil recovery method, particularly a thermal method such as steam injection, so as to sweep both the more permeable and less permeable oil saturated intervals in a formation in a uniform manner.

SUMMARY OF THE INVENTION 1 have discovered that it is possible to treat a subterranean viscous petroleum-containing formation which is comprised of two or more intervals or zones having significantly difl'erent permeabilities to fluid flow in such manner that a thermal fluid such as steam introduced into the formation will move through both the more permeable and the less permeable streaks uniformly, thereby recovering petroleum from both zones. The method requires the establishing of separate injection means between the surface and each of the petroleum saturated intervals. For example, a single well bore may be used for this purpose in a formation containing two petroleum saturated intervals. The production tubing may be in fluid communication with the lower zone with a packer set between the zones and the annular space between the tubing and the well casing utilized as a second fluid communication means between the surface and the upper petroleum saturated zone.

If the oil recovery fluid to be utilized is steam, then steam injection is established into the more permeable zone first, in order to establish a pressure gradient in that zone which will preclude the movement of fluids from the less permeable zone into the more permeable zone, Afier steam injection has been established into the more permeable zone, a hydrocarbon solvent such as propane, natural gasoline, or other aliphatic hydrocarbons is introduced by the separate communication means into the less permeable zone. From about 0.05 to 0.5 pore volumes of hydrocarbon solvent is introduced into the less permeable zone after which it is displaced with any convenient gaseous drive fluid such as natural gas. Steam is then injected into the pretreated low permeability zone in order to displace the hydrocarbon solvent away from the injection well. Initially, the steam injection pressure into the low permeability zone is maintained at a value less than the steam injection pressure in the high permeability zone. After the solvent has been displaced through the formation to the production well, the steam injection pressure in the low permeability zone may be increased until the injection pressure in both zones is essentially equalized.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing illustrates in cross sectional view, a subterranean petroleum formation containing two zones of significantly different permeability, into which formation an injection well is completed with separate communication means established between each zone and the surface of the earth for treatment according to the process of my invention.

DETAILED DESCRIPTION OF THE lNVENTlON The preferred method of employing the process of my invention can best be understood by referring to the figure, in which there is shown a tar sand deposit consisting of two horizontally oriented bituminous petroleum saturated sand layers, layer 1 having a permeability of approximately 800 millidarcies and located there below, bituminous petroleum saturated interval 2 having a permeability of 350 millidarcies. interposed between layers 1 and 2 is impermeable shale streak 3 which separates oil sand interval 1 and 2. The separation is incomplete, since there are a number of openings in the shale streak 4 which establishes vertical communication between zones 1 and 2, respectively. Packer 8 is positioned between the two intervals in the unperforated portion of the casing 9. Production tubing extends inside tubing 9 to a point adjacent perforation 7, so that tubing 10 is in fluid communication with the lower, low permeability oil sand zone 2 and the surface of the earth. The annular space 11 between tubing 10 and easing 9 defines a separate flow path between the surface of the earth and the upper, high permeability interval 1. On the surface, steam generator 12 is in fluid communication with annular space 11, and also is connected through valve 13 to the production tubing 10. Tank 14 contains the hydrocarbon solvent to be introduced into the lower, low permeability oil sand interval. Pump 15 serves to inject the hydrocarbon solvent material into the lower, low permeability interval at a controlled rate. Storage tank 16 contains a supply of an inert gas such as nitrogen, with appropriate lines connecting to both the tubing 10 and the annular space 11, through valves 17 and 18.

In the application of the process of my invention to the well equipped in accordance with attached drawing, the first phase involves introduction of steam into the upper, high permeability strata 1 through annular space 11. In subterranean formations containing appreciable gas permeability, steam may be introduced directly into the formation without any pretreatment. In very low permeability formations, particularly tar sand deposits which have very low initial fluid transmissitivity, a preliminary sweep with an inert gas is highly desirable in order to establish gas saturation in the formation prior to the introduction of steam into the formation. For this purpose, the inert gas such as nitrogen from supply tank 16 is introduced through opened valve 17 into the annular space 11, where it passes through perforations 6 into the upper, high permeability formation 1. Preferably, the inert gas is introduced until an indication of gas production is obtained at a remotely located production well. This will establish gas saturation in upper petroleum saturated sand interval 1, which will facilitate the subsequent introduction of steam thereinto.

After the upper petroleum saturated sand interval 1 has been saturated with an inert gas, valve 17 is closed and steam from generator 12 is introduced into the an nular space 11, from which it passes through perforations 6 into the upper, high permeability strata 1. This is continued until the pressure in upper strate has increased, and may be continued until there is an indication at a remotely located production well of the passage of steam thereinto, such as by the increase in that well s temperature.

After steam injection has been initiated into upper, high permeability interval 1, the next step involves the introduction of a hydrocarbon material into lower, low permeability interval 2. This is accomplished by closing valves 13 and 18, opening valve 19 and activating pump 15 so as to pump the hydrocarbon substance from storage tank 14 into production tubing 10 where it passes through perforation 7 into the lower, low permeability petroleum-saturated interval 2. As above, it is sometimes preferable in very low permeability formations to precede the hydrocarbon injection with a quantity of inert gas, and this may be accomplished as above by passing nitrogen or other inert gas into the production tubing for a period of several days prior to the initiation of hydrocarbon injection. If the permeability of the formation is sufficient to permit pumping hydrocarbon into the formation directly, the inert gas sweep step may be eliminated.

Any low molecular weight aliphatic hydrocarbon having from 1 to 10 carbon atoms may be utilized as the hydrocarbon solvent material in accordance with the process of my invention. Gaseous hydrocarbons, e. g. methane, ethane, propane and butane may be used. Unsaturated hydrocarbons may also be utilized, although ordinarily economics would preclude the use of such olefinic hydrocarbons. Carbon dioxide may also be used. Normally liquid hydrocarbons, e.g. butane, pentane, heptane, hexane, octane, nonane or decane may be used effectively. Aromatic hydrocarbons such as benzene, toluene, or xylene are also effective for this purpose. Mixtures of two or more of these hydrocarbon materials or a mixture of hydrocarbon and carbon dioxide may also be used effectively. Commercially available mixtures such as naptha, natural gasoline, kerosene, and mixed aromatic-paraffinic hydrocarbon materials such as are sometimes available as waste streams of refinery operations are also very suitable materials for use in the process of my invention.

One preferred embodiment of the process of my invention involves the injection of a gaseous hydrocarbon solvent into the lower, low permeability zone 2 followed by the introduction of a liquid hydrocarbon solvent material. The gaseous material is more readily displaced into the zone, and opens up the permeability of the zone so that the subsequently injected liquid hydrocarbon material may be pumped into the petroleum saturated zone. For example, from 0.05 to 0.5 pore volumes of a naturally gaseous material such as propane may be pumped into the low permeability zone, after which a normally liquid solvent such as natural gasoline is injected to displace the propane and further open up the permeability of the originally low permeability zone.

As a general rule, the amount of solvent to be introduced into the formation should be in a range from about 0.05 to about 0.5 pore volumes based on the area to be swept by the injected fluid in the particular pattern being employed.

For the purpose of this application, by the term zones of widely differing permeabilities" it is meant that the permeability of one earth formation layer is at least 0.25 percent greater than the permeability of another adjacent earth formation layer.

During the period that solvent is being injected into the low permeability interval, steam injection is maintained in the high permeability interval. This is important so as to maintain a pressure in the high permeability interval greater than the pressure in the low permeability interval so in the event there is vertical communication between the high and low permeability intervals, the hydrocarbon treating fluid will be confined by pressure gradients to the low permeability interval.

After the solvent injection phase is completed, steam injection may then be initiated into the low permeability interval while maintaining steam injection into the high permeability interval. Initially the steam injection pressure into the low permeability interval is maintained at a lower value than the steam injection pressure for the high permeability interval. The reason for maintaining the steam injection pressure into the low permeability interval is to ensure that the steam injected into the low permeability interval does not channel through vertical communication points into the upper communication interval. Steam pressure into the low permeability interval may be slowly increased until the injection pressure of steam being introduced into the high permeability interval is essentially the same as the steam pressure being introduced into the low permeability interval.

The above described process has been described in terms of a steam flooding operation, although the same pretreatment process may be used in application of in situ combustion or hot water flooding to subterranean formations where there are encountered two or more zones of widely differing permeabilities. The process may be applied to subterranean formations containing relatively low viscosity crudes which are to be subjected to water flooding operations or to other supplemental recovery operations, or to other supplemental recovery operations involving the injection of an unheated aqueous fluid into the formation for the purpose of displacing petroleum toward the production well. For example, adjacent oil saturated intervals of widely varying permeabilities which are to be subjected to surfactant flooding may also be pretreated according ot the process of this invention, by the introduction of a solvent material into the low permeability strata while maintaining the positive pressure in the high permeability strata in order to confine the hydrocarbon treating material to the low permeability strata.

Other variations of the process of my invention include heating the inert gas and/or the solvent prior to introduction thereof into the low permeability zone of the formation. The gas and/or solvent may be heated to a temperature of from about l00F to about 350F.

The process of my invention may be more readily understood by reference to the following field example,

which is offered only for purpose of illustration and disclosure and is not intended to be limitative or restrictive of my invention.

F lELD EXAMPLE A tar sand deposit located under an overburden thickness of 240 feet was penetrated by a dually completed injection well. A production well was located 25 feet from the injection well. Although this was intended as a monitor well, it was completed for oil production, and the well was complete open hole throughout the full thickness of the tar sand deposit. The tar sand interval was comprised of two separate zones, the upper zone being approximately 30 feet in thickness and the lower zone approximately 40 feet in thickness. A shale stringer separated the two zones although it was known that there was vertical communication between the two zones. The upper zone was the more permeable zone. The well was completed essentially as shown in the attached drawing, with the tubing being in fluid communication with the lower, low permeability zone and the annular space establishing fluid communication between the surface and the upper, more permeable zone. The permeability of the upper zone was about 250 millidarcies and the permeability of the lower zone was about millidarcies.

The oil recovery process being employed in this particular field was a controlled, low temperature combustion process accomplished by the introduction of a mixture of steam and air into the formation. The ratio of steam to air was approximately 0.4 barrels of steam (as water) per million standard cubic feet of air. Nitrogen was injected slowly into the upper formation first, to establish a gas saturation between the injection well and the monitor well located 25 feet away. Steam and air were then injected into the formation, and injection was continued until the temperature in the monitor well showed an increase, indicating that heated fluid had saturated the upper, more permeable zone.

After steam-air injection was established into the upper zone, approximately barrels of propane was introduced via the production tubing into the lower zone while maintaining steam-air injection into the upper zone. The propane was displaced with approximately 150 barrels of natural gasoline. After injection of both the propane and natural gasoline, steam and air injection was initiated into the lower zone, maintaining the injection pressure into the lower zone substantially lower than the injection pressure in the upper zone. Steam and air were injected into the lower zone using the same steam-air ratio as given above, but at the lower pressure. The injection pressure into the lower interval was thereafter raised gradually until, after about three days, the injection pressure was the same in both intervals.

A frothy, bituminous petroleum-water mixture was produced at the remotely located well and a very satisfactory rate, and it appeared that production was being obtained from both intervals indicating that equalization of receptivities of the two zones to steam and air had been accomplished.

While my invention has been described in terms of a number of illustrative embodiments, it is not limited since other variations thereof will be apparent to persons skilled in the related art without departing from the true spirit and scope of my invention. Similarly, while a mechanism has been proposed to explain the benefits resulting from the application of the process of my invention, it is not necessarily represented hereby that this is the only or even the principal mechanism re sponsible for the resultant benefits, and it is not my intention to be bound by any particular explanation of the operation of the process of my invention. It is my desire that my invention be limited and restricted only by those limitations and restrictions that appear in the claims appended hereinafter below.

I claim:

1. A method of recovering petroleum from the subterranean petroleum containing formation, said petroleum formation comprising at least two intervals of widely differing permeabilities, the method being of the type wherein a recovery fluid is introduced into one well for the purpose of increasing the mobility of the formation petroleum and formation petroleum is recovered from a remotely located production well, wherein the improvement for increasing the vertical conformance comprises:

a. establishing a separate communication path between the surface of the earth and at least two zones of the subterranean formation whose permeabilities differs from one another;

b. introducing the recovery fluid into the more permeable zone until the pressure has been increased in that zone to a value greater than the original formation pressure;

c. introducing a solvent selected from the group consisting of hydrocarbons having from 1 to 10 carbon atoms, carbon dioxide and mixtures thereof into the zone of lower permeability;

d. thereafter introducing the recovery fluid into the lower permeability zone to displace the solvent away from the injection well, originally maintaining the injection pressure into the lower permeability zone at a value less than the injection pressure of the drive fluid being introduced into the zone of greater permeability; and

e. recovering petroleum from the remotely located well.

2. A method as recited in claim 1 wherein from about 0.05 to about 0.5 pore volumes of solvent are introduced into the formation.

3. A method as recited in claim 1 wherein the recovery fluid is steam.

4. A method as recited in claim 1 wherein the recovery fluid is a mixture of air and steam.

5. A method as recited in claim 1 wherein the recovery fluid is hot water.

6. A method as recited in claim 1 wherein the solvent is gaseous.

7. A method as recited in claim 1 wherein the solvent is liquid.

8. A method as recited in claim 1 wherein first a gaseous solvent is introduced into the zone of lower permeability, after which a liquid solvent is introduced into the same zone.

9. A method as recited in claim 1 wherein the injection pressure of recovery fluid being introduced into the zone of lower permeability is gradually increased until it is essentially equal to the injection pressure at which the recovery fluid is being introduced into the zone of higher permeability.

10. A method of recovering petroleum from a subterranean, petroleum containing formation penetrated by an injection well and a production well of the type wherein drive fluid is introduced into one well to displace petroleum toward a second well, said formation consisting of at least two zones of widely different permeabilities, wherein the improvement for increasing the vertical conformance of the oil recovery method comprises:

a. establishing separate injection means between the surface and each of the zones of differing permea bility,

b. introducing the aqueous drive fluid into the zone of higher permeability,

c. introducing a solvent for the formation petroleum selected from the group consisting of aliphatic or aromatic hydrocarbons having from 1 to 10 carbon atoms, carbon dioxide and mixtures thereof, into the zone of lower permeability,

d. thereinafter introducing the drive fluid into the zone of lower permeability to displace the hydrocarbon material away from the injection well, initially maintaining the drive fluid injection pressure below the pressure at which the drive fluid is being injected into the zone of higher permeability, and

e. recovering petroleum from the production well.

11. A method as recited in claim 10 comprising the additional step of increasing the drive fluid injection pressure in the low permeability zone until the drive fluid is being injected at substantially equal pressures into the low permeability zone and the high permeability zone. 

1. A METHOD OF RECOVERING PETROLEUM FROM THE SUBTERRANEAN PETROLEUM CONTAINING FORMATION, SAID PETROLEUM FORMATION COMPRISING AT LEAST TWO INTERVALS OF WIDELY DIFFERING PERMEABILITIES, THE METHOD BEING OF THE TYPE WHEREIN A RECOVERY FLUID IS INTRODUCED INTO ONE WELL FOR THE PURPOSE OF INCREASING THE MOBILITY OF THE FORMATION PETROLEUM AND FORMATION PETROLEUM IS RECOVERED FROM A REMOTELY LOCATED PRODUCTION WELL, WHEREIN THE IMPROVEMENT FOR INCREASING THE VERTICAL CONFORMANCE COMPRISES: A. ESTABLISHING A SEPARATE COMMUNICATION PATH BETWEEN THE SURFACE OF THE EARTH AND AT LEAST TWO ZONES OF HE SUBTERRANEAN FORMATION WHOSE PERMEABILITIES DIFFERS FROM ONE ANOTHER, B. INTRODUCING THE RECOVERY FLUID INTO THE MORE PERMEABLE ZONE UNTIL THE PRESSURE HAS BEEN INCREASED IN THAT ZONE TO A VALUE GREATER THAN THE ORIGINAL FORMATION PRESSURE, C. INTRODUCING A SOLVENT SELECTED FROM THE GROUP CONSISTING OF HYDROCARBONS HAVING FROM 1 TO 10 CARBON ATOMS, CARBON DIOXIDE AND MIXTURES THEREOF INTO THE ZONE OF LOWER PERMEABILITY, D. THEREAFTER INTRODUCING THE RECOVERY FLUID INTO THE LOWER PERMEABILITY ZONE TO DISPLACE THE SOLVENT AWAY FROM THE INJECTION WELL, ORIGINALLY MAINTAINING THE INJECTION PRESSURE INTO THE LOWER PERMEABILITY ZONE AT A VALUE LESS THAN THE INJECTION PRESSURE OF THE DRIVE FLUID BEING INTRODUCED INTO THE ZONE OF GREATER PERMEABILIEY AND E. RECOVERING PETROLEUM FROM THE REMOTELY LOCATED WELL.
 2. A method as recited in claim 1 wherein from about 0.05 to about 0.5 pore volumes of solvent are introduced into the formation.
 3. A method as recited in claim 1 wherein the recovery fluid is steam.
 4. A method as recited in claim 1 wherein the recovery fluid is a mixture of air and steam.
 5. A method as recited in claim 1 wherein the recovery fluid is hot water.
 6. A method as recited in claim 1 wherein the solvent is gaseous.
 7. A method as recited in claim 1 wherein the solvent is liquid.
 8. A method as recited in claim 1 wherein first a gaseous solvent is introduced into the zone of lower permeability, after which a liquid solvent is introduced into the same zone.
 9. A method as recited in claim 1 wherein the injection pressure of recovery fluid being introduced into the zone of lower permeability is gradually increased until it is essentially equal to the injection pressure at which the recovery fluid is being introduced into the zone of higher permeability.
 10. A method of recovering petroleum from a subterranean, petroleum containing formation penetrated by an injection well and a production well of the type wherein drive fluid is introduced into one well to displace petroleum toward a second well, said formation consisting of at least two zones of widely different permeabilities, wherein the improvement for increasing the vertical conformance of the oil recovery method comprises: a. establishing separate injection means between the surface and each of the zones of differing permeability, b. introducing the aqueous drive fluid into the zone of higher permeability, c. introducing a solvent for the formation petroleum selected from the group consisting of aliphatic or aromatic hydrocarbons having from 1 to 10 carbon atoms, carbon dioxide and mixtures thereof, into the zone of lower permeability, d. thereinafter introducing the drive fluid into the zone of lower permeability to displace the hydrocarbon material away from the injection well, initially maintaining the drive fluid injection pressure below the pressure at which the drive fluid is being injected into the zone of higher permeability, and e. recovering petroleum from the production well.
 11. A method as recited in claim 10 comprising the additional step of increasing the drive fluid injection pressure in the low permeability zone until the drive fluid is being injected at substantially equal pressures into the low permeability zone and the high permeability zone. 